1. Field of the Invention
The present invention relates to an insulating composition for a substrate, and a prepreg and a substrate using the same.
2. Description of the Related Art
With the advance of electronic devices, printed circuit boards are becoming lighter, thinner, and smaller day by day. In order to meet these requirements, wiring of a printed circuit board is becoming more complicated and highly-densified. Therefore, electrical, thermal, and mechanical stability of a substrate serve as important factors. Among them, particularly, the coefficient of thermal expansion (CTE) is one of the important factors that affect reliability in manufacture of the substrate.
A printed circuit board chiefly comprises copper serving as circuit wiring and a polymer serving as an interlayer insulator. The CTE of the polymer constituting an insulation layer is much higher than that of copper. In order to overcome this difference, the CTE of the polymer constituting the insulation layer is reduced by impregnating the polymer into a woven glass fiber or adding an inorganic filler to the polymer.
Generally, as the amount of the inorganic filler is increased, the CTE of the insulating layer is reduced, but there is a limit in reducing the CTE of the insulating layer indefinitely due to a manufacturing process of the substrate.
Further, in order to meet the requirement for highly-densified fine patterns, surface roughness of the insulating layer is also considered as an important factor. The size of the inorganic filler added in order to secure the surface roughness is gradually reduced. However, as the size of the inorganic filler is reduced, the problem with the uniform dispersibility of the inorganic filler is on the rise, and, particularly, the problem that the nanoscale filler must be uniformly dispersed is also on the rise.
FIG. 1 shows a structure of a printed circuit board which comprises copper serving as circuit wiring and a polymer serving as an interlayer insulating layer. The CTE of the copper (Cu) circuit wiring is 10 to 20 ppm/° C., and the CTE a1 of a typical polymer material used in the insulating layer is 50 to 80 ppm/° C. Since the CTE of the polymer is greatly increased above a glass transition temperature (Tg, 150 to 200° C.), the CTE a2 at high temperature reaches 150 to 180 ppm/° C.
Further, heat is rapidly supplied to a PCB for 3 to 5 seconds at a temperature of about 280° C. when mounting components such as semiconductors on the PCB. At this time, cracks of a circuit formed by plating and deformation of the substrate may occur due to a big difference in the CTE between the circuit and the insulating layer.
Ultimately, a polymer material of the insulating layer, which has a CTE equal to those of copper as circuit wiring and semiconductor chips placed on the substrate, is required. However, materials obtained by adjusting the kind and amount of a polymer and an inorganic filler, which constitute an existing insulating layer, are difficult to satisfy the requirement for complicated and highly-densified wirings of the printed circuits.
Meanwhile, there are two types of polymer composite insulating materials which are used in the insulating layer for a printed circuit board. One is a prepreg prepared by impregnating the polymer composite insulating layer into a woven glass fabric or a woven glass cloth to semi-cure the polymer composite insulating layer at a temperature below a glass transition temperature Tg of the material as in FIG. 2.
The other is a film manufactured using only the polymer composite insulating material without including the woven glass fabric as in FIG. 3. The latter method blends a polymer composite insulating material, an inorganic filler, a hardener, a solvent, additives, a curing accelerator, and so on at an optimal blending ratio and mixes, disperses, and casts the blend to form a film.
A main polymer composite insulating material, which forms an insulating layer of a conventional printed circuit board, is an epoxy resin. The CTE of the epoxy resin itself is about 70 to 100 ppm/° C. In order to reduce the CTE of the epoxy resin, the epoxy resin is impregnated into a woven glass fiber or a large amount of inorganic filler with a low CTE are added to an epoxy matrix to implement a low CTE as shown in FIG. 4.
The CTE of the epoxy resin is linearly reduced in proportion to the amount of the added fillers. However, when a large amount of the inorganic filler are added to reduce the CTE, dispersibility of the inorganic filler in the matrix is greatly deteriorated so that aggregation of the filler occurs and surface roughness of the printed circuit board is much increased. Further, since viscosity of the epoxy is rapidly increased, there are many difficulties in forming products. Especially, in case of a product having a multilayer structure such as an insulating film used in the printed circuit board, there are many cases where interlayer bonding is impossible.
For these limitations, it is required to reduce the CTE of the epoxy resin itself and simultaneously improve an effect by introducing a critical amount of inorganic filler which can secure lamination processability. For example, the epoxy resins having different structures are mixed to reduce the CTE of the epoxy resin itself. At this time, the component and composition of each epoxy resin are important.
Further, since the CTE of the epoxy resin is greatly affected by the kind, size, and shape of the inorganic filler as well as the amount of the inorganic filler, miniaturization, that is, nanoscaling of the added inorganic filler is required to implement a hyperfine pattern. However, although a nanoscale inorganic filler is added, it is still difficult to obtain a homogeneous film through uniform dispersion of the filler.
Therefore, the development of a material of an insulating layer of a printed circuit board with a low CTE is needed. Further, as thinning of a substrate is in progress, a substrate with increased strength and rigidity is needed. The development of a material of an insulating layer which satisfies these two characteristics is needed.